The present invention relates to the field of calcining of minerals such as talc, kaolin, and the like. In particular it relates to processes, substances, and apparatus that can enhance and achieve production of a product in a manner that achieves enhanced results for the product and the end uses of the product, such as predictably and repeatable generating a specific L*a*b* value or perhaps otherwise afford technical or economic advantages.
Numerous industries which produce plastics of various types, paints for multiple uses, elastomerics for roof and other coatings, paper, caulking and other products use minerals which serve as fillers, extenders and opacifiers, and which perform functions with the base materials which cannot occur without such fillers, extenders or opacifiers.
One such mineral is talc. Talc is commonly called hydrous magnesium silicate or magnesium silicate hydroxide, with the chemical formula Mg3(Si4O10)(OH)2. Silicon oxide tetrahedra are composed of three oxygen molecules which form the base of tetrahedron located at the surface of a sheet. The fourth oxygen molecule in the lower plane forms the opposing apex, with a silicon being located within the tetrahedron. These internal sheets are bonded together by Van Der Waals forces. Some tetrahedra are reversed and have an active apex located at the surface of the sheet, this apex consisting of a hydroxyl group (OH) bonded to the silicon of the tetrahedron to form a silanol group SiOH. The surface sheets are connected together to the same internal sheets by Van Der Waals forces.
Upon heating, perhaps beginning at approximately 100° C., talc (here commonly termed a “feed”) may undergo a series of significant changes in the chemical nature and crystalline structure. At about 100° C., free moisture may be evolved from talc. Then, as further heating occurs, various organic compounds may be released and the carbonate impurities associated with talc may be driven off perhaps beginning at about 550° C. Additional heating may cause the crystalline water to evolve at about 950° C. Heating beyond 1000° C. (commonly called hard calcination) may be practiced to ensure conversion of the talc material to secondary, distinct materials, commonly referred to as “products” such as enstatite (MgSiO2), protoenstatite (MgSiO2), clinoenstatite (MgSiO3), or the like. In the presence of some impurity materials, diopside (CaMg(SiO3)2) may be formed. Such impure materials may cause formation of products such as diopside, dolomite (calcium magnesium carbonate), chlorite (hydrated magnesium aluminum silicate), limestone (calcium carbonate), or the like.
Calcining of talc has been regarded as a simple process, but discoveries disclosed herein have shown how new and remarkable results and products may be achieved, including but not limited to, use of low value talc that can be transformed into a higher value material. Past technologies have formed ceramic pigments from talc and have created hydrophyllic substances. Past talc calcinations patents may have generally focused on reaching a single temperature within a calciner, and other temperature measurements are taken as resultant of the firing temperature. Other past technologies have used direct-fired calcination in rotary kilns, which may feature a flame at the discharge end, with the hot combustion gases moving toward the feed end, taking with it high volumes of feed powder, or if pelletized, taking the abraded powder off the pellets and moving it to bag houses or other methods of dust capture. The discovery disclosed in embodiments of the present invention include the effect of the speed of gas evolution in the first heating stages, the removal of said gases, and perhaps even the impact on morphology of the final product. Past technologies have shown the use of calcining temperatures from about 900° C. to about 1275° C. and may have also shown the formation of enstatite and clinoenstatite when calcining. The formation of those products is not the subject of this invention, but the methods to achieve various quality parameters in the final product is a key subject. It is no longer necessary to accept blindly whatever product comes out of the calciner—the products can be engineered, perhaps beginning at the feed end.
The key quality parameters sought by firms now calcining talc, in addition to retaining the lamellarity (platyness) of talc, are the L*a*b* values, hue, chromaticity, the surface area, specific gravity, and refractive index of the pigments produced by calcinations.
The thermal conversion of feed such as talc to products such as enstatite (between about 1100° C. to about 1220° C.) may provide products which are no longer talc, and may or may not have properties of talc. For example, talc is hydrophobic, while enstatite is hydrophyllic. Depending on the conditions under which the transformation occurs, product softness or hardness, platy morphology, density, refractive and other properties may be impacted. Chemically, in addition to the above, inert siloxane bridges may be replaced by active hydrophyllic groups. X-ray diffraction patterns may demonstrate that the former talc substance is no longer talc.
Talc has for many years been used in the production of various ceramics, and researchers have found that heating (e.g., calcining) to temperatures in the about 1200° C. range changes the soft, platy crystals to ceramic, elongated crystals that can be sharp edged and having Mohs hardness in the 5 to 7 range.
The products developed using embodiments of the present invention have many uses, including but not limited to, a partial substitution for the use of titanium dioxide in paints, plastics, elastomers, caulks and sealants, and other products. The ceramic pigments (e.g., enstatite, clino-enstatite, or the like) produced may also be used for specialty high temperature greases and lubricants, as well as exotic ceramics such as is found in space shuttle tiles or in the proppants used in hydraulic fracturing of horizontal gas and oil wells. This may be a desirable partial substitution because talc feed and its products may be more cost effective to utilize than the current or existing technology.
In the past, calcining may involves heating a tube or igniting natural gas, propane, fuel oil, or other hydrocarbon materials. The mineral to be calcined, such as talc, may be fed into the heated tube. In the past, the tube generally consisted of one heat zone, but may have a slightly cooler heat zone at the feed end due to the heat exchange which is occurring between the cold feed, the environment, and the calcining tube. Gasses which are given off during the chemical conversion of feed to products have been pulled off with combustion gasses in such a manner as to maintain the proper heat in the tube. Feed into the calciner can be milled, or may be of larger generally uniform size. Moreover product can also be milled when required, but clearly the cost of milling soft feed talc is significantly less than much harder ceramic product.
Commercial use of talc and its calcined products are limited by the color, as assessed by the CIE L*a*b* scale, the size and size distribution, and other mineral specific properties. For example, if adequate brightness (L* value of perhaps 98) is not obtained, the product materials may not be utilized in paints. If perhaps the a* value, which measures red/green color is >about 0.3 or generally considered ‘red’ the secondary product materials may not be utilized in caulks. If perhaps the b* value, which measures blue/yellow is >about 2.8 or generally considered too yellow, the secondary materials may be undesirable for use in plastics.
Therefore, commercial use of raw talc is often limited by the L*, a* and/or b* values. For example talc naturally occurs in multiple colors such as grey, black, green, pink and white. Black or grey talc can, upon calcining, become white, in the range from L*=about 93 or higher, depending on calcining conditions. This may or may not be sufficiently bright to be utilized in paint. Thus, even if it may be less expensive to use than titanium dioxide, it may likely not be available to serve as a substitute.
Past technologies may include U.S. Pat. No. 3,366,501 by Lamar, U.S. Pat. No. 5,229,094 by Clauss, FR Pat. No. 2,585,691 by Ducasse, U.S. Pat. No. 5,154,766 by Young, U.S. Pat. No. 5,371,051 by Pope, and U.S. Pat. No. 3,309,214 by Podschus, each incorporated by reference herein in its entirety.
U.S. Pat. No. 3,366,501 by Lamar may provide a method of producing a white ceramic enstatite pigment from talc, which may be hydrophilic and can be used for paper and paint manufacturing. The L*a*b* values are not mentioned or discussed. Lamar may utilize a direct-fired calciner and the product may milled to a particle size d50 of 6 microns or larger to produce a saleable powder.
U.S. Pat. No. 5,229,094 by Clauss may describe talc undergoing chemical or thermal transformation wherein the talc particles may be heated to a temp below 900° C. perhaps under conditions to avoid conversion of talc to enstatite.
Patent FR 2,585,691 A1 by Ducasse may describe a method of preparing a fine lamellar calcined talc powder perhaps in a direct-fired rotary kiln from a talc feed first milled to d50 of 10 microns and then milled again to d50 of 3.5 microns.
U.S. Pat. No. 5,154,766 by Young may provide raw kaolin that may be slurried with the addition of some magnetite, flocculated with aluminum and sulfuric acid, perhaps even run through a filter press to produce a cake which was dried. However, the product of the Young patent may not be acceptable as a talc-enstatite product and therefore cannot be considered useful.
U.S. Pat. No. 5,371,051 by Pope et. al. may attempt to increase the opacity of calcined kaolin pigments perhaps by the addition or retention of small percentages of titanium dioxide. This method may not be practical due to the additional cost of titanium dioxide.
U.S. Pat. No. 3,309,214 by Podschus may provide double calcinations of kaolin, first in a shock calciner at 400° C. to 500° C. for one second whereby the turbulent hot gas stream may partially change the crystal structure, and then in a second calciner of varying types for about one hour at about 800° C. to 1100° C. This process may have achieved some improvement over competitive kaolin processes but the improvement does not translate to talc processing. The vertical turbulent shock calciner may not provide sufficient residence time to be beneficial to color, or to completely calcine talc to enstatite.
In the past, controlling the production of the L*a*b* values has not been commercially feasible because calcination and the end products may appear to be a somewhat random event, perhaps resulting in unpredictable products which may have unpredictable L*a*b* values. Moreover, mineral qualities such as platyness (or lamellarity), opacity, softness, strength, or the like may affect the commercial value and may also be considered somewhat uncontrolled as secondary products are produced via calcination of minerals. By contrast, embodiments of the present invention addresses and demonstrates the beneficial impact of controlling the calcination process and systems.